Common polymers, such as polystyrene or polyethylene, theoretically comprise extended linear chains of monomers. However, such polymers can also have branches; see Graessley et al., Macromolecules, Vol. 9, No. 1, 1976, p 127. Such branched polymers often have significantly better processing characteristics (especially when the number of monomer units in the branch chain exceeds 100 to 300 units) than their linear or short chain branched counterparts. For example, the melt strength of a long chain branched polymer can be significantly higher than the melt strength of its linear or short chain branched counterpart of the same molecular weight and often shear thin to a greater extent, see Macosko, Rheology—Principles, Measurements, and Applications, pages 497-506. Polymers that exhibit higher melt strength have superior processing properties and can command a higher price.
Polymer characterization is an important branch of chemical analysis. Characterization of a polymer to determine its topology, (in other words, the degree and type of branching of the polymer) is currently insufficient for correlating molecular structure to physical properties. Nuclear Magnetic Resonance (NMR) analysis can determine the average number of branch points per polymer molecule; see DePooter, et al., J. App. Pol. Sc., 42, p 399-408 (1991). However, such an NMR analysis does not determine the molecular weight distribution of the long chain branches or the type of branching, for example, “T” branching, “star” branching, “comb” branching and “T” branching.
Polymers have been characterized by Field Flow Fractionation (FFF) by flowing a solution of a polymer in a channel perpendicular to a force field (such as a centrifugal force field in a centrifuge) to separate the components of the polymer in successive elution volumes from the channel. See, for example, Janca, Field-Flow Fractionation—Analysis of Macromolecules and Particles, 1988, Marcel Dekker. In FFF, higher molecular weight fractions of the polymer generally elute from the channel after the lower molecular weight fractions of the polymer. FFF has not apparently been used to characterize polymers for long chain branching topology.
Ionic polymers, such as sulfonated polystyrene, have been characterized by electrophoresis (EP) in a system where the polymer is dissolved in a buffer solution and migrated under the influence of an electric field (electrophoretic mobility) through a medium such as a gel swelled with the buffer. Lower molecular weight fractions of the polymer migrate faster than higher molecular weight fractions of the polymer. The characterization of long chain branching of ionic polymers has been attempted using EP but without success. See Smisek and Hoagland, Science, 8 Jun. 1990, p 1221-1223 and especially page 1222, third column, which stated: “We next compared the dependence of [electrophoretic] mobility on N [molecular weight] for linear and star PSS [linear and star branched sulfonated polystyrene] (FIG. 4). Surprisingly, over the molecular size range displayed [N from about 100 to about 100,000], the mobility depended only on N, and was independent of molecular topology”, in other words, no separation of linear from branched polymer was observed.
Hydrodynamic Chromatography (HDC) is an important polymer characterization technique. See, for example, Small, J. Colloid Interface Science, 1974, 48, p 147 and Stegeman et al., J. Chrom., 1993, 657(2), p 283-303. In HDC a solution of a polymer is flowed by an eluant over the surfaces of non-porous beads packed in column (or through a capillary column). In HDC the higher molecular weight fractions of the polymer elute from the column before the lower molecular weight fractions of the polymer. More accurately, HDC separates components of a polymer according to their hydrodynamic size in a solution or a dispersion. However, HDC has not apparently been used to characterize polymers for long chain branching topology.
Size Exclusion Chromatography (SEC) (also called Gel Permeation Chromatography (GPC)) is an important polymer characterization technique. See, Yau et al., Modern Size-Exclusion Liquid Chromatography, 1979, John Wiley & Sons. In SEC a solution of a polymer is flowed by an eluant through a column packed with porous beads. The polymer diffuses into and out of the porous beads (there being essentially no flow of the eluant through the porous beads because the flow channels around the beads are significantly larger than the pores of the beads). In SEC the higher molecular weight fractions of the polymer elute from the column before the lower molecular weight fractions of the polymer. More accurately, SEC separates components of a polymer according to their hydrodynamic size.
A branched polymer has a somewhat smaller radius of gyration in solution than a linear polymer of the same type and molecular weight. Thus, SEC can be used to characterize a polymer for branching. See Drott and Mendelson, Journal of Polymer Science, Part A-2, Vol. 8, 1970, p 1361. However, as pointed out by Drott and Mendelson, as the degree of branching of a polymer increases the relative effect on SEC elution volume decreases. Furthermore, SEC provides no direct information of the shape of the molecule (for example, star shape or H shape) or the molecular weight of the branch. Thus, the information obtained from SEC for the study of long chain branching of polymers is not sufficient to define the Theological properties of the polymer. It would be a clear advance in the art if a better solution were discovered for the problem of characterizing a polymer for long chain branching.